A top Bank of England official will Monday face a grilling from lawmakers over his role in the rate-rigging scandal engulfing British banks, in an appearance that could make or break his chance of being the central bank’s next governor.

Paul Tucker, the BOE’s deputy governor for financial stability, faces parliament’s Treasury Select Committee at 1530 GMT as part of lawmakers’ probe into banks’ alleged manipulation of the London Interbank Offered Rate, or Libor, before and during the financial crisis.

Mr. Tucker asked to appear to give his side of the story after Barclays let slip that in 2008 a senior BOE official–soon identified as then-markets chief Mr. Tucker–had passed on concerns emanating from somewhere in government about Barclays’ apparently high cost of funding in a call to Bob Diamond, who quit as CEO last week amid the rate-rigging furor.

Barclays employees somehow interpreted this call as tacit approval for the bank to lower its Libor submissions, which feed into the overall rate. The charge is Barclays subsequently misrepresented its true borrowing costs.

To be sure, Mr. Diamond took some of the heat off Mr. Tucker when he appeared before the committee last week. He said Mr. Tucker wasn’t approving anything, tacit or otherwise, but instead was warning him that someone in the-then Labour government, or possibly the civil service, was alarmed at Barclays’ apparent ill health. Mr. Diamond said he feared then-Prime Minister Gordon Brown would nationalize the lender, he told lawmakers.

Mr. Diamond’s evidence sent accusations flying in Westminster about who knew what about Libor-rigging and when, and half-forgotten Labour advisers and ministers quickly took to the airwaves to say they had nothing to do with it and never spoke to Mr. Tucker.

Chancellor of the Exchequer George Osborne and his opposite number in the Labour party, former Brown adviser Ed Balls, traded insults after Mr. Osborne suggested Mr. Balls had questions to answer over the affair.

So Mr. Tucker can expect to be asked just who was it that prodded him to make that fateful call to Mr. Diamond in October 2008. The answer could be explosive. He will also have to explain just what the BOE knew or suspected about the role of Libor in the crisis.

Mr. Tucker, whose sometimes rumpled appearance belies his extensive experience in financial markets and regulatory affairs, is the BOE’s leading internal candidate to replace Governor Mervyn King next year. His testimony today will play a pivotal role in keeping his hopes of taking the top job alive.

If he leaves the Treasury Select Committee with any doubts as to his probity or competence, his career at the BOE may well have reached its summit already.

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